The temperature of a condensation surface, at which the rate of the condensate exactly equals the evaporation, is frequently defined as the dew point temperature or saturation point. Optical condensation type dew point hygrometers, commonly called “chilled mirror” hygrometer, are configured to detect a dew point or saturation point of a gas and are well known in the art of analytical instrumentation. These “chilled mirror” hygrometers are usually configured so the condensation surface is maintained in vapor pressure equilibrium with the analyzed gas.
Referring to FIG. 1, one of the characteristics of the known “chilled mirror” hygrometers is that a light source 4 and a photodetector 8 of these “chilled mirror” hygrometers are on the same side of the mirror 2 as the sample 6. The surface of the mirror 2 in direct contact with the analyzed gas is called a condensation surface 1. In other words, the light from the light source 4 of the “chilled mirror” hygrometer travels through the analyzed gas 6, reflects off the condensation surface 1 of the mirror 2, and travels through the analyzed gas 6 to the photodetector 8. The mirror 2 is usually a metal plate of a highly reflective metal such as gold, rhodium or platinum. In the known “chilled mirror” hygrometers the analyzed gas 6 is in direct contact with the condensation surface 1 of the mirror 2, the light source 4 and the photodetector 8.
In the known “chilled mirror” hygrometers the mirror 2 is usually cooled by a thermoelectric or Peltier cooler 10 until dew or frost starts to condense on the condensation surface 1 of the mirror 2. While the Peltier cooler 10 manipulates the temperature of the mirror 2, the “chilled mirror” hygrometer continuously monitors the condensation surface 1 of the mirror 2 to detect condensation. The temperature of the condensation surface 1 is usually measured with a thermocouple or thermistor 12 embedded in the mirror 2. When the condensate starts to form on the condensation surface 1, the condensate on the condensation surface 1 scatters the light from the light source 4 and reduces the amount of light, detected by the photodetector 8. At the point in time when the photodetector 8 detects the change in the intensity of the light reaching its surface, the “chilled mirror” hygrometer records the temperature of the mirror 2. This recorded temperature is a dew point of the analyzed gas 6.
One of the problems of the known “chilled mirror” hygrometers is that when the analyzed gas 6 is corrosive, the mirror 2 and the other elements of the known “chilled mirror” hygrometer quickly deteriorate from contact with the analyzed gas 6. The other problem is that in many applications where dew point measurements are required the presence of a metal from the metal plate mirror 2 in the test environment is highly undesirable.
The light source 4 and the photodetector 8 are usually insulated from the analyzed sample by epoxy sealants or other protective vapor barriers, such as Mylar® film. However, if the analyzed gas is highly corrosive, these elements of the hygrometer quickly deteriorate. Also the metal plate mirror 2 is in direct contact with the analyzed gas 6, which leaks metal into the tested environment and is unacceptable in many applications.